Field of the Invention
The present invention relates to an interferometer system, a lithography apparatus, and an article manufacturing method.
Description of the Related Art
There has been known an interferometer system including an interferometer that measures the position of an object to be measured using laser light emitted from a laser light source. In the interferometer system, the laser light emitted from the light source is firstly guided to the interferometer after splitting by a beam splitter or after folding by a mirror. Next, the interferometer separates the laser light into measuring light and reference light, and detects an interference signal generated by superposition of the measuring light returned by reflection from the object to be measured and the reference light. Consequently, the difference in displacement between the measuring light and the reference light is derived, so that the position of the object to be measured can finally be specified. Here, when any one of temperature, pressure, and humidity changes in an optical path through which either the measuring light or the reference light passes, the refractive index of air changes. Thus, a measurement error may occur by the difference between the measuring light and the reference light both of which have been adversely affected due to a change in the refractive index of air. Accordingly, in order to reduce the occurrence of measurement error, there is also an interferometer system that is provided with a cover on an optical path or is provided with a device which performs local air-conditioning control. In contrast, in the common optical path, through which the superposed measuring light and reference light pass, from the light source to the interferometer, it has been conventionally thought that no measurement error caused by a change in refractive index itself which is changed by variations in temperature or the like occurs in principle. This is because, since the optical path is common to both measuring light and reference light, no optical path length difference occurs if both the measuring light and the reference light are subject to the same change. However, when three hours or greater is taken for measurement, the impact of a nonlinear error caused by a change in pressure or the like may not be ignored even by such a laser interferometer. Accordingly, Japanese Patent Laid-Open No. H11-44506 discloses an interferometer measuring apparatus that seals an optical path with a sealing pipe such that a common optical path is not affected by the exterior environment, particularly, pressure.
However, even when an optical path is sealed with a sealing pipe so as not to be affected by a change in refractive index as disclosed in Japanese Patent. Laid-Open No. H11-44506, a measurement error may occur in a short-time measurement which is substantially immune to pressure variations or the like if a portion open to atmosphere is present in at least a part of the common optical path. This is because, since the refractive index distribution occurs in the common optical path and the interface between airs having different refractive indices constantly changes upon the occurrence of airflow, the angle of laser light (optical axis) also changes with a change in interface. When the angle of laser light changes in the optical path, the irradiation position of measuring light on the object to be measured changes by an amount proportional to the distance from the position at which a change in angle occurs to the object to be measured. FIGS. 7A and 7B are reference diagrams illustrating a measurement error for comparison. Here, if a surface 40 to be measured and measuring light 41 are perfectly orthogonal to each other as shown in FIG. 7A, a measurement error E1 occurs as the approximate L(1−cos θ) from the relationship between an optical path length L and an angle change θ. In contrast, when the surface 40 to be measured is offset from the measuring light 41 by an offset θ2 from the orthogonal relationship as shown in FIG. 7B, an additional measurement error E2 occurs as the product of L and tan θ1 tan θ2. In this case, examples of factors for causing a change in refractive index in the common optical path include a change in temperature caused by a laser light source. The temperature of the laser light source is typically controlled by using a heater in order to stabilize the wavelength of laser light. Thus, the laser light source itself is a large heat-generating source. In particular, a large temperature distribution is formed in the vicinity of an emission opening for emitting laser light. In other words, even when an optical path is sealed with a sealing pipe as disclosed in Japanese Patent Laid-Open No. H11-44506, a measurement error may occur due to a change in refractive index interface caused by temperature change of the laser light source if a gap which is open to atmosphere is present between the sealing pipe and the laser light source.